1. Field of the Invention
The present invention relates to a wavelength measuring apparatus for measuring wavelength of light under measurement and in particular to wavelength measurement apparatus for measuring wavelength of sweep light that varies continuously.
2. Description of the Related Art
Conventionally, an interferometer is used to measure the wavelength of light under measurement. FIG. 11 shows a Michelson interferometer. The Michelson interferometer 1100 comprises a reference light source 101 for emitting reference light having a known wavelength xcex0, a fixed mirror 1102, a movable mirror 1103 provided slidably in parallel with the optical path, a half mirror 1104 provided at an angle of 1045 degrees from the optical path, a photo-detector for light under measurement 1105, and a photo-detector for reference light 1106.
In the Michelson interferometer 1100, light under measurement having an unknown wavelength xcex is emitted toward Point B of the half mirror 1104. Part of the outgoing light under measurement is reflected at the right angle at Point B of the half mirror 1104, reversed by 180 degrees in direction by the fixed mirror 1102, passes through Point A of the half mirror 1104, and incident on the photo-detector for light under measurement 1105. Other part of the light under measurement passes through Point B of the half mirror 1104, reversed by 180 degrees in direction by the movable mirror 1103, reflected at the right angle at Point A of the half mirror 1104, and incident on the photo-detector for light under measurement 1105.
Meanwhile, part of reference light emitted from the reference light source 1101 is reflected at the right angle at Point A of the half mirror 1104, reversed by 180 degrees in direction by the fixed mirror 1102, passes through Point B of the half mirror 1104, and incident on the photo-detector for reference light 1106. Other part of the light under measurement passes through Point A of the half mirror 1104, reversed by 180 degrees in direction by the movable mirror 1103, reflected at the right angle at Point B of the half mirror 1104, and incident on the photo-detector for reference light 1106.
In this way, on each photo-detector 1105, 1106 are incident light that passed through the fixed mirror 1102 and light that passed through the movable mirror 1103 thus generating interference between these light beams. Thus, in case the movable mirror 1103 is slide in the direction of the arrow in the figure, output signals output from the photo-detectors include cyclic peaks caused by interference as shown in FIG. 12.
The pitch length P of the photo-detector for light under measurement 1105 corresponds to the wavelength xcex of the light under measurement. In case the movable mirror 1103 is moved for a predetermined distance D, the wavelength xcex of the light under measurement is determined from the number of peaks n0 of the output signal from the photo-detector for light under measurement 1105, the number of peaks n1 of the output signal from the photo-detector for reference light 1106, and the wavelength xcex0 of the reference light, and represented by the following expression:
xcex=(n0/n1)xc3x97xcex0xe2x80x83xe2x80x83(1) 
However, in a related art interferometer such as a Michelson interferometer, it is assumed that the wavelength of the light under measurement is fixed during measurement. Thus it was impossible to accurately measure the wavelength in case the wavelength of the light under measurement continuously varied. That is, the number of peaks n1 does not reflect local variation of the wavelength of the light under measurement so that the average value of varied wavelengths is measured in case the wavelength of the light under measurement varies while the movable mirror 1103 is slid.
An object of the invention is to provide wavelength measurement apparatus that can measure the wavelength of the light under measurement under sweep process with high accuracy and in real time even in case the wavelength is continuously swept.
In order to attain such an object, according to a first aspect of the invention, there is provided a wavelength measurement apparatus comprising:
an optical filter (such as a fiber-optic Etalon 3 in FIG. 1) to which a light beam is incident;
a photo-detector (for example a photodiode 4 in FIG. 1) for detecting the transmitted light of the optical filter, the photo-detector for outputting intensity of the transmitted light;
a counter (for example a counter 8 in FIG. 1) for counting the number of peaks of the output of the photo-detector to generate a count value; and
a controller (for example a CPU 14 in FIG. 1) for calculating the wavelength of the light beam based on the count value of the counter.
Here, the optical filter may be any optical filter that selectively transmits light having a predetermined length and may be composed of an interference optical filter where a multi-layered optical film is evaporated on Fabry-Perot Etalon, silica based glass, or silicon.
In the first aspect of the invention, the light under measurement is incident on the optical filter. The optical filter selectively transmits light having a predetermined waveform. The photo-detector detects the transmitted light that passed through the optical filter and outputs the light intensity of the transmitted light. In case the wavelength of the light under measurement is continuously swept, the measured is transmitted through the optical filter each time the wavelength of the light under measurement satisfies predetermined conditions that conform to the physical characteristics of the optical filter.
The predetermined wavelength interval (finesse) is a length determined according to the physical characteristics of the optical filter so that it is possible to know the correct value in advance based on a theoretical formulae such as the Airy""s formulae or a measured value. Thus, the count value (number of peaks) currently counted by the counter represents a relative variation of the wavelength from the start of sweep to this point in time. The controller calculates the wavelength of the light under measurement based on the count value so that it can calculate the instantaneous wavelength value at this point in time. As a result, it is possible to measure the wavelength of the light under measurement under sweep process with high accuracy and in real time even in case the wavelength is continuously swept.
According to a second aspect of the invention, there is provided a wavelength measurement apparatus according to the first aspect of the invention, the controller resets the count value when light having a known reference wavelength is incident.
In the second aspect of the invention, an operator resets the count value via the controller when light having a known reference wavelength is incident. Accordingly, the count value of the counter while the wavelength of the light under measurement is being swept represents a relative variation from the reference wavelength. Thus, the controller can accurately calculate the wavelength of light under measurement under sweep. It is thus possible to measure the wavelength of the light under measurement under sweep process with high accuracy and in real time even in case the wavelength is continuously swept.
According to a third aspect of the invention, the wavelength measurement apparatus further comprises:
a synchronization signal output unit (for example a comparison register 13 in FIG. 1) for outputting a synchronization signal with a predetermined timing,
wherein the controller acquires the count value each time the synchronization signal from the synchronization signal output unit is detected.
In the third aspect of the invention, the synchronization signal output unit outputs a synchronization signal with a predetermine timing. The controller acquires the count value each time the synchronization signal from the synchronization signal output unit is detected. The controller calculates the wavelength per count value acquired by the controller. Thus, it is possible to calculate in real time the wavelength of the light under measurement per predetermined timing in the process of sweeping the light under measurement. It is also possible to calculate, correct and display the wavelength based on the count values after capturing the count values.
The synchronization signal output unit preferably outputs the synchronization signal based on the count value of the counter, as in a fourth aspect of the invention.
As in a fifth aspect of the invention, the light under measurement is emitted from a tunable light source (for example a TLS 1 in FIG. 1) comprising a light source and a wavelength adjusting mechanism (for example a motor/encoder 2 in FIG. 1) for varying the wavelength of the light source. The synchronization signal output unit preferably outputs the synchronization signal based on the operation amount of the wavelength adjusting mechanism.
According to a sixth aspect of the invention, the counter counts the number of peaks of the output by incrementing or decrementing the count value each time the output of the photo-detector exceeds/drops below a predetermined reference value.
According to the sixth aspect of the invention, the counter counts the number of peaks of the output by incrementing or decrementing the count value each time the output of the photo-detector exceeds/drops below a predetermined reference value. Thus it is made easy to accurately calculate the number of peaks of the output of the photo-detector.
According to a seventh aspect of the invention, the reference value varies according to the light intensity of the reference light branched from the light under measurement before the optical filter.
According to the seventh aspect of the invention, the reference value varies according to the light intensity of the reference light branched from the light under measurement before the optical filter. Thus, even in case a fluctuation is present in the intensity of the light under measurement, it is possible to avoid an error in the count value caused by the fluctuation. That is, the fluctuation exerts an influence on both the output of the photo-detector and the reference value so that it is possible to cancel the fluctuation in case these values are compared with each other.
According to an eighth aspect of the invention, wherein the controller corrects the wavelength value of the light beam calculated by the controller based on at least one of the sweep start wavelength value and the sweep end wavelength value of the light beam.
The sweep start wavelength value and the sweep end wavelength value are preferably measured to a maximum accuracy through measurement using for example a wavemeter with sufficient accuracy guaranteed.
According to the eighth aspect of the invention, the controller corrects the wavelength value of the light under measurement calculated by the controller based on at least one of the sweep start wavelength value and the sweep end wavelength value of the light under measurement. In case an error is present in the calculation results of the controller, the error can be reduced. It is obvious that the wavelength value of the light under measurement can be corrected based on both the sweep start wavelength value and the sweep end wavelength value. In such a case, the accuracy of wavelength halt of the light under measurement (xc2x1xcex94WL) at the start and end of sweep can be set to zero thus the error in the peak interval can be set to zero. This obtains a more accurate wavelength value.
According to a ninth aspect of the invention, the optical filter is a fiber-optic Etalon (for example fiber-optic Etalon 3 in FIG. 1) composed of an optical fiber and high-reflection members (for example high-reflection films 3a, 3a) supplied on both ends of the optical fiber.
In the ninth aspect of the invention, when light under measurement is incident on the fiber-optic Etalon, the light under measurement repeats reflection between high-reflection member at one end and the high-reflection member at the other end in the fiber-optic Etalon. When the wavelength of the incident light under measurement satisfies specific conditions, the light under measurement is transmitted through the fiber-optic Etalon. In case the wavelength of the incident light under measurement is continuously swept, the light intensity of the transmitted light output by the photo-detector reaches a peak per predetermined wavelength interval. Here, the predetermined wavelength interval (xcex94xcex) is a length determined by the physical characteristics of the fiber-optic Etalon. Assuming that the length of the fiber-optic Etalon as L, refraction index as n, and the wavelength of the light under measurement as xcex, xcex94xcex is represented by the following expression (2):
xcex94xcex=xcex2/(2 nL)xe2x80x83xe2x80x83(2) 
As shown in the expression (2), the wavelength interval (xcex94xcex) is inversely proportional to the length L of the fiber-optic Etalon. The shorter the wavelength interval (xcex94xcex), the better the resolution of wavelength variation during sweep. For the fiber-optic Etalon, it is possible to provide the sufficient length L so that it is easy to measure the wavelength of continuously swept light under measurement with accuracies of for example 1 pm to 0.1 pm or better.
According to a tenth aspect of the invention, wherein the wavelength measurement apparatus further comprises a heat insulator for keeping the temperature of the fiber-optic Etalon to be constant.
According to the tenth aspect of the invention, the heat insulator keeps constant the temperature of the fiber-optic Etalon so that it is possible to prevent expansion/contraction of the length L of the fiber-optic Etalon caused by variation in the ambient temperature. This assures more accurate measurement of the wavelength of light under measurement.
According to an eleventh aspect of the invention, there is provided a wavelength measurement apparatus comprising:
a first optical filter (for example a gas cell in FIG. 7) to which one of light beams branched is incident and through which the one of light beams is transmitted, the first optical filter for discriminating the one of the light beam with at least two of pre-calibrated wavelength components;
a second optical filter (for example a fiber-optic Etalon in FIG. 7) to which the other of the branched light beam is incident and the incident light beam is transmitted in a free spectral range shorter than the interval between the two of the pre-calibrated wavelength components; and
a controller (for example a CPU 119 in FIG. 7) for calculating the wavelength of the light beam based on the transmitted light of the first optical filter and the transmitted light of the second optical filter.
In the eleventh aspect of the invention, the light under measurement is incident while branched to the first optical filter and the second optical filter. The first optical filter discriminates between at least two types of pre-calibrated wavelength components. It is thus possible to set reference values on at least two points based on the intensity of the transmitted light of the first optical filter. The second optical filter has a free spectral range shorter than the interval between these at least two types of pre-calibrated wavelengths. In case the wavelength of the light under measurement is swept continuously, a plurality of peaks are present in the intensity of the transmitted light of the second optical filter between the two types of wavelengths. It is thus possible to accurately correct the wavelength interval in the free spectral range of the second optical filter based on the number of peaks and the interval between the reference values on two points. Thus, the controller can accurately calculate the relative variation of the wavelength from start of sweep to the present point in time, so that it is possible to measure the wavelength of the light under measurement under sweep process with high accuracy even in case the wavelength is continuously swept.
According to a twelfth aspect of the invention, wherein the controller corrects the free spectral area of the second optical filter by dividing the wavelength scale in the variation between the two types of wavelengths by the number of peaks of the transmitted output of the second optical filter.
According to the twelfth aspect of the invention, correction between two types of wavelengths assures accurate correction in the wavelength interval of the second optical filter also in wavelengths outside the two types of wavelengths.
According to a thirteenth aspect of the invention, the first optical filter is a gas cell (for example a gas cell in FIG. 7) for absorbing at least two types of pre-calibrated wavelength components.
According to the thirteenth aspect of the invention, the gas cell absorbs at least two types of pre-calibrated wavelength components. Thus it is possible to set a reference value with a timing when the intensity of the transmitted light of the gas cell is significantly weak. The gas cell has an excellently stable wavelength discrimination characteristic against disturbance such as temperatures thus assuring more accurate measurement of light under measurement.
According to a fourteenth aspect of the invention, the first optical filter is an Etalon (for example Fabry-Perot Etalon in FIG. 10) that assumes the interval between at least two types of pre-calibrated wavelengths as a free spectral range.
According to the fourteenth aspect of the invention, cyclic peaks are generated in the intensity of the transmitted output of Etalon while light under measurement is being swept. Thus it is possible to set a reference value with a timing the peak is generated.
According to a fifteenth aspect of the invention, the wavelength measurement apparatus further comprises:
a counter (for example a counter 114 in FIG. 7) for counting the number of peaks of transmitted output of the second optical filter; and
wherein the controller resets the count value of the counter when light having a known reference wavelength is incident.
In the fifteenth aspect of the invention, an operator resets the count value via the controller when light having a known reference wavelength is incident. Accordingly, the count value of the counter while the wavelength of the light under measurement is being swept represents a relative variation from the reference wavelength. Thus, the controller can accurately calculate the wavelength of light under measurement under sweep. It is thus possible to measure the wavelength of the light under measurement under sweep process with high accuracy and in real time even in case the wavelength is continuously swept.
According to a sixteenth aspect of the invention, the wavelength measurement apparatus further comprises a synchronization signal output unit (for example a comparison register 17 in FIG. 7) for outputting a synchronization signal with a predetermined timing,
Wherein the controller acquires the count value each time the synchronization signal from the synchronization signal output unit is detected.
Here, the synchronization signal output unit preferably outputs a synchronization signal based on the count value of the counter according to the fifth aspect of the invention, as in a seventeenth aspect of the invention.
According to an eighteenth aspect of the invention, the light under measurement is emitted from a tunable light source (for example a TLS 1 in FIG. 7) comprising a light source and a wavelength adjusting mechanism (for example a motor/encoder 102 in FIG. 7) for varying the wavelength of the light source, the synchronization signal output unit preferably outputs a synchronization signal based on the operation amount of the wavelength adjusting mechanism.
In the eighteenth aspect of the invention, the synchronization signal output unit outputs a synchronization signal with a predetermine timing. The controller acquires the count value each time the synchronization signal is detected. The controller calculates the wavelength per count value acquired by the controller. Thus, it is possible to calculate the wavelength of the light under measurement per predetermined timing in the process of sweeping the light under measurement. It is also possible to calculate, correct and output the wavelength based on the count values after capturing the count values.
According to a nineteenth aspect of the invention, the counter counts the number of peaks of the transmitted output by incrementing or decrementing the count value each time the transmitted output of the second optical filter exceeds/drops below a predetermined reference value.
According to the nineteenth aspect of the invention, the counter counts the number of peaks of the transmitted output by incrementing or decrementing the count value each time the transmitted output of the second optical filter exceeds/drops below a predetermined reference value. Thus it is made easy to accurately calculate the number of peaks of the output of the photo-detector.
According to a twentieth aspect of the invention, the reference value varies according to the light intensity of the reference light branched from the light under measurement before the second optical filter.
According to the twentieth aspect of the invention, the reference value varies according to the light intensity of the reference light branched from the light under measurement before the optical filter. Thus, even in case a fluctuation is present in the intensity of the light under measurement, it is possible to avoid an error in the count value caused by the fluctuation. That is, the fluctuation exerts an influence on both the transmitted output of the second optical filter and the reference value so that it is possible to cancel the fluctuation in case these values are compared with each other.
According to a twenty-first aspect of the invention, the controller corrects the wavelength value of the light under measurement calculated based on at least one of the sweep start wavelength value and the sweep end wavelength value of the light under measurement.
The sweep start wavelength value and the sweep end wavelength value are preferably measured to a maximum accuracy through measurement using for example a wavemeter with sufficient accuracy guaranteed.
According to the twenty-first aspect of the invention, the controller corrects the wavelength value of the light under measurement calculated by the controller based on at least one of the sweep start wavelength value and the sweep end wavelength value of the light under measurement thus an error can be reduced. It is obvious that the wavelength value of the light under measurement can be corrected based on both the sweep start wavelength value and the sweep end wavelength value. In such a case, the accuracy of wavelength halt of the light under measurement (xc2x1xcex94WL) at the start and end of sweep can be set to zero thus the error in the peak interval can be set to zero. This obtains a more accurate wavelength value.
According to a twenty-second aspect of the invention, the second optical filter is a fiber-optic Etalon (for example fiber-optic Etalon 108 in FIG. 7) comprising an optical fiber and high-reflection films (for example high-reflection films 8a, 8a) supplied on both ends of the optical fiber.
In the twenty-second aspect of the invention, when light under measurement is incident on the fiber-optic Etalon, the light under measurement repeats reflection between the high-reflection member at one end and the high-reflection member at the other end in the fiber-optic Etalon. When the wavelength of the incident light under measurement satisfies specific conditions, the light under measurement is transmitted through the fiber-optic Etalon. In case the wavelength of the incident light under measurement is continuously swept, the light intensity of the transmitted light output by the photo-detector reaches a peak per predetermined wavelength interval. Here, the predetermined wavelength interval (xcex94xcex) is a length determined by the physical characteristics of the fiber-optic Etalon. Assuming that the length of the fiber-optic Etalon as L, refraction index as n, and the wavelength of the light under measurement as xcex, xcex94xcex is represented by the following expression (2):
xcex94xcex=xcex2/(2 nL)xe2x80x83xe2x80x83(2) 
As shown in the expression (2), the wavelength interval (xcex94xcex) is inversely proportional to the length L of the fiber-optic Etalon. The shorter the wavelength interval (xcex94xcex), the better the resolution of wavelength variation during sweep. For the fiber-optic Etalon, it is possible to provide the sufficient length L so that it is easy to measure the wavelength of continuously swept light under measurement with accuracies of for example 1 pm or better.
According to a twenty-third aspect of the invention, the wavelength measurement apparatus further comprises a heat insulator for keeping constant the temperature of the fiber-optic Etalon.
According to the twenty-third aspect of the invention, the heat insulator keeps constant the temperature of the fiber-optic Etalon so that it is possible to prevent a change in the length L and the refraction index n. This assures more accurate measurement of the wavelength of light under measurement.